data wanted;
set aaa;
if no ne lag(no) then z=1;
else z+1;
run;

data aaa;
input no a1$ b1 c1;
cards;
1 x1 1 1
1 z1 2 1
1 y1 1 2
2 x2 1 2
2 y2 1 2
3 x3 2 2
;
run;
proc sort data=aaa out=b;
by a1;
run;
data c(rename=(no=z));
set b(keep=no);
run;
data bb;
merge aaa c;
run;

厉害啊，赞个，学习了！